Tag metrics

Tag metric is a function which enables us to order tags. Technically, it can be alphabetical ordering but it isn't really useful conceptually. More convenient metric is _popularity_ of a tag, which stands for amount of resources labeled with the tag.

With _linear selector_ we can use only 1 metric. But with _cloud selector_ we can use up to 3 metrics (see ARK-Builders/ark-components-android#7).

Possible metrics to implement:
* _popularity_ — amount of resources standing behind a tag
* _freshness_ — the latest date of modification among all resources standing behind a tag
* _coupling_ — amount of common resources a tag has with other tags
* _branching_ — amount of tags crossing given tag

It also should be possible to invert ordering by any metric.

Start by locating the linear and cloud selector implementations, then compare their current tag-ordering behavior. Clarify which metrics are in scope, how each is computed, and how inversion should work; done means the selected metrics order tags consistently in both selectors, with tests covering the behavior.

Written by the indexing model from the issue text.